Many viruses form stable self-assembled protein cages that function to store, protect and transport nucleic acid. We have previously shown that the cage structure of viruses can be used as constrained reaction vessels for the encapsulation and release of a wide range of materials other than its native RNA genome. In this way virus protein cages can be thought of as nanometer sized containers able to encapsulate other molecules through well-defined chemical interactions. The current proposal will explore the use of these virus cage structures for encapsulation and targeted delivery of therapeutic agents as well as development of these cages as magnetic resonance imaging contrast agents based on our demonstrated ability to engineer the coat protein. The principle objective of this proposal is to develop a model viral system for the use of virus cage structures in the high- density packing and release of therapeutic materials (molecules and polymers). Packaging within the virus can be driven by electrostatic complementarily between the inner protein interface and the relevant therapeutic material(s). One objective will be to extend the range of therapeutic materials that can be entrapped within the viral protein cage by engineering the electrostatic properties of the inner surface of the protein cage. A second major objective is to develop viral protein cages as potential magnetic resonance imaging contrast agents by engineering the inherent metal binding sites on the virion for binding 180 molecules of the paramagnetic Gd(III) ion. A third major objective is to express peptide 11 from the laminin protein on the outer surface of the virion and to determine its effectiveness at specifically targeting viral cages to cells expressing laminin-binding protein. A fourth major objective is to utilize inherent structural transitions in the virion to engineer new well defined chemical switches (based on redox potential and pH) to induce gating for selective entrapment and release of therapeutic materials. Virion gating results in the reversible opening/closing of 60 separate 20Angstrom units holes in the protein cage. We propose to use site-directed mutagenesis to engineer disulfide linkages and altered pH gating switches at these pores and test for their ability to entrap and release therapeutic materials.